When coating workpieces such as vehicle bodies or their parts with paint or other coating materials, such as sealants or adhesives, it is known that the most accurate metering possible of the coating material supplied to the applicator is necessary. Metering is performed on demand, i.e. the volumetric flow (quantity flowed per unit of time) of the coating material supplied to the applicator during coating must be adjustable as a function of the specific sub-areas of the workpiece with great precision and short response times, where the individual setpoints are saved in the master controls for the installation and are specified thereby.
The requirements for metering systems for coating installations are in practice substantial and to some extent hard to implement, primarily the requirements for accuracy which in many cases must be absolute and with respect to metering fluctuations be at the least ±1% from the setpoint, with a high degree of reproducibility during fluctuations in temperature, viscosity and pressure. Because of the required accuracy, stepless volume control is preferably required. The components of the metering system must, among other things, be free of dead space to prevent curing. Special requirements apply when metering particular coating materials, such as NAD material (non-aqueous polymer dispersion), for which special measuring devices are required, or when high metering pressure is reached during the application of certain materials, up to 400 bar for example in the case of PUR. Different conditions apply with respect to volumetric flow, the quantity flowed, which in typical cases can be between 2 and 50 ccm/sec, for example. Additional requirements relate to the acceptable system rise and reaction times (<40 ms until reaching ±5% of the setpoint), freely programmable adjustability of feed pressure with low reaction time (<100 ms) and automatic, dynamic adjustment of feed pressure with changes in coating material viscosity, the possibility for automatic calibration with material changes and low delay times at the start of operations. Generally, not only the costs for the installation and its maintenance but the weight and dimensions of the system components should be as low and small as possible, in particular with respect to assembly in or on application robots.
Various metering systems with continuous or discontinuous metering of the coating material are known for coating installations. Continuous metering systems have advantages in principle such as relatively small expenditure (low costs), continuous material flow, wide metering range, short cycle times or refill times and compact dimensions. However, known continuous metering systems are too inaccurate for many applications. They may contain pressure regulators with simple control loops with which only pressure regulation or, by using a flow metering cell, volume control can be performed, or flow regulators in whose control loops, for example, control valves may be used as actuator and flow metering cells as actual value transducers. Apart from their relatively low metering accuracy, these metering systems are intrinsically relatively slow to react to changes in setpoints, which, for example, noticeably diminishes coating quality when applying adhesive beads or when sealing seams because of the skips when they switch on and off, particularly at the beginning and end of the beads, but also with changes in volume on the applied bead. In addition, continuous metering systems which meter volumetrically using geared metering pumps are known and customary. Discontinuous metering systems on the other hand typically contain piston metering units which are known as designs for single or dual metering units with electrical servo metering drive and which can operate without a closed control loop, but which are praticably controlled pressure-dependent. A pressure regulator may be conveniently installed upstream of the metering system to ensure as constant an inlet pressure as possible.
With regard to the prior art, reference is made to Dürr/Behr Technical Manual 02/1994 “Color Volume Control”; DE 38 22 835; DE 691 03 218 T2; DE 100 65 608; EP 1 287 900; EP 1 314 483; EP 1 346 775; EP 1 474 161; and patent applications EP 05 111 273 dated Nov. 24, 2005 and DE 10 2005 042 336.1 dated Sep. 6, 2005
With this as the point of departure, there is a need for simple continuous metering with great metering accuracy and short reaction delays.